The invention generally relates to a sensor assembly.
Seismic surveying may be used for purposes of obtaining characteristics and attributes of an oil or gas reservoir. For a land-based seismic survey, a seismic source produces acoustic waves, which travel downwardly into the earth and are reflected back to a number of seismic sensors, called geophones. The geophones produce signals, which indicate the detected seismic waves, and the signals from the geophones may be recorded and processed to yield information about the nature of the earth below the area being investigated.
One type of geophone, called a single coil geophone, includes a single coil of wire that is suspended in an internal magnetic field (a field formed from one or more permanent magnets within the geophone, for example). Movement of the coil relative to the internal magnetic field due to a seismic wave results in cutting lines of magnetic flux, an event that produces a corresponding output voltage (across the coil) that indicates the seismic wave.
The single coil of the single coil geophone has a relatively small mass, which makes it relatively easy to control. However, a conventional single coil geophone may be relatively sensitive to magnetic fields that are produced by sources that are external to the geophone, such as overhead electrical power transmission lines, electrical power lines associated with an electric railroad and an underground pipeline protection system. More specifically, external magnetic fields may cause unintended movement of the geophone's coil, which may impart a significant noise component to the geophone's output voltage. Another type of geophone (called a dual coil geophone) has a second coil in a design that ideally diminishes the effects of external magnetic fields at the price of increasing the weight of the coil form. A microelectromechanical (MEM)-based geophone may also be relatively insensitive to external magnetic fields, as this geophone typically does not contain any explicit inductive-type elements (such as a coil), which are affected by an external magnetic field. However, both dual coil and MEM-based geophones typically are considerably more expensive than their single coil counterpart, and the dual coil geophone's higher coil mass is more difficult to control if used as part of a feedback circuit.